Failure diagnosis device of emission control system

ABSTRACT

In a failure diagnosis device of an emission control system that utilizes an electrode-based PM sensor provided downstream of a particulate filter in an exhaust conduit to diagnose a failure of the particulate filter, disclosed embodiments may suppress reduction of accuracy of diagnosis of a failure due to in-cylinder rich control. The failure diagnosis device of the emission control system starts application of a predetermined voltage to electrodes of the electrode-based PM sensor after a sensor recovery process that removes PM depositing between the electrodes of the PM sensor, and diagnoses a failure of the particulate filter based on an output of the PM sensor measured after elapse of a predetermined time period since the start of application of the predetermined voltage. The failure diagnosis device performs the sensor recovery process during in-cylinder rich control or triggered by termination of the in-cylinder rich control and subsequently performs a measurement process after termination of the in-cylinder rich control and the sensor recovery process.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2015-058558, filed on Mar. 20, 2015,which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a technique of diagnosing a failure ofan emission control system including a particulate filter that is placedin an exhaust conduit of an internal combustion engine and morespecifically relates to a technique of diagnosing a failure of aparticulate filter by utilizing an electrode-based particulate matter(hereinafter “PM”) sensor that is placed downstream of the particulatefilter in the exhaust conduit.

DESCRIPTION OF THE RELATED ART

An existing sensor for detecting the amount of PM (particulate matter)included in exhaust gas is an electrode-based PM sensor that includeselectrodes arranged to he opposed to each other across an insulatinglayer and utilizes an electrical characteristic that the value ofelectric current flowing between the electrodes is varied according tothe amount of PM depositing between the electrodes.

An existing method of diagnosing a failure (abnormality) of aparticulate filter by utilizing an electrode-based PM sensor compares anoutput value of the PM sensor (electric signal relating to value ofelectric current flowing between electrodes of PM sensor) at the timewhen a predetermined time period has elapsed since termination of aprocess of removing PM depositing between the electrodes of the PMsensor (hereinafter referred to as “sensor recovery process”) with apredefined reference value, and diagnoses that the particulate filterhas a failure when the output value of the PM sensor is higher than thepredefined reference value.

SUMMARY

An emission control system of an internal combustion engine includes athree-way catalyst or an NSR (NO_(X) storage reduction) catalyst, inaddition to a particulate filter. This emission control system mayperform a process of supplying a non-combusted fuel component (forexample, hydrocarbon (HO) to the three-way catalyst or the NSR catalystto convert NO_(X) stored in or adsorbed to the NSR catalyst or toproduce ammonia (NH₃) by the three-way catalyst or the NSR catalyst(rich spike process). A technique of the rich spike process controls theair-fuel ratio of an air-fuel mixture that is subjected to combustion ina cylinder of the internal combustion engine to a rich air-fuel ratiowhich is lower than a stoichiometric air-fuel ratio (hereinafterreferred to as “in-cylinder rich control”).

As the result of intensive experiments and examinations, disclosedembodiments may improve performance based on the finding that the outputvalue of the PM sensor (value of electric current flowing betweenelectrodes) may be lower in the case where the rich spike process by thein-cylinder rich control is performed than in the case where the richspike process is not performed on the assumption that a fixed amount ofPM flows into the PM sensor. Accordingly, performing the rich spikeprocess by the in-cylinder rich control in the predetermined time periodleads to the possibility that the output value of the PM sensor afterelapse of the predetermined time period becomes lower than thepredefined reference value even when the particulate filter has afailure. This may lead to inaccurate diagnosis that the particulatefilter that actually has a failure is diagnosed to have no failure.

By taking into account the above problems, in a failure diagnosis deviceof an emission control system that utilizes an electrode-based PM sensorprovided downstream of a particulate filter in an exhaust conduit todiagnose a failure of the particulate filter, disclosed embodiments maysuppress reduction of accuracy of diagnosis of a failure due toin-cylinder rich control.

A failure diagnosis device of an emission control system, in accordancewith the present disclosure, may commence application of a predeterminedvoltage to electrodes of an electrode-based PM sensor after a sensorrecovery process that oxidizes and removes PM depositing between theelectrodes of the PM sensor, and diagnoses a failure of the particulatefilter based on an output of the PM sensor measured after elapse of apredetermined time period since the start of application of thepredetermined voltage. The failure diagnosis device performs the sensorrecovery process during in-cylinder rich control or triggered bytermination of the in-cylinder rich control. This enables a measurementprocess to be performed at an earlier time after termination of thein-cylinder rich control.

More specifically, according to one aspect of the disclosure, there isprovided a failure diagnosis device of an emission control system thatis applied to the emission control system comprising a particulatefilter that is placed in an exhaust conduit of an internal combustionengine and is configured to trap PM in exhaust gas; an exhaust gaspurification device that is placed upstream of the particulate filter inthe exhaust conduit and is configured to purify the exhaust gas byutilizing a non-combusted fuel component included in the exhaust gas;and a supplier configured to perform in-cylinder rich control ofchanging an air-fuel ratio of an air-fuel mixture subjected tocombustion in the internal combustion engine to a rich air-fuel ratiowhich is lower than a stoichiometric air-fuel ratio, so as to supply thenon-combusted fuel component to the exhaust gas purification device. Thefailure diagnosis device comprises a PM sensor that is provided todetect an amount of PM flowing out of the particulate filter, the PMsensor having a sensor element that includes electrodes opposed to eachother across an insulating layer and a heater that is configured to heatthe sensor element, the PM sensor being configured to output an electricsignal relating to an amount of PM depositing between the electrodesunder application of a predetermined voltage to the sensor element; anda controller comprising at least one processor configured to perform aprocess of diagnosing a failure of the particulate filter, based on anoutput value of the PM sensor. The controller configured to perform asensor recovery process that controls the heater to heat the sensorelement to a temperature that allows for oxidation of PM and therebyoxidizes and removes the PM depositing between the electrodes, and ameasurement process that starts application of the predetermined voltageto the sensor element after termination of the sensor recovery processand measures an output value of the PM sensor when a predetermined timeperiod has elapsed since start of application of the predeterminedvoltage; and diagnose a failure of the particulate filter by comparingthe obtained output value of the PM sensor with a predefined referencevalue. The controller performs the sensor recovery process during thein-cylinder rich control or triggered by termination of the in-cylinderrich control and subsequently performs the measurement process aftertermination of the in-cylinder rich control and the sensor recoveryprocess. The “electric signal relating to the value of electric currentflowing between the electrodes” herein may be a value of electriccurrent flowing between the electrodes or may he a resistance valuebetween the electrodes.

The occurrence of a failure such as breakage or erosion in part of aparticulate filter increases the amount of PM that slips through theparticulate filter. This increases the amount of PM adhering ordepositing between the electrodes of the PM sensor in the predeterminedtime period in the case where the particulate filter has a failure,compared with the case where the particulate filter has no failure. As aresult, the resistance value between the electrodes at the time when thepredetermined time period has elapsed since start of application of thepredetermined voltage to the electrodes of the PM sensor (hereinafterreferred to as “reading timing”) is lower in the case where theparticulate filter has a failure than in the case where the particulatefilter has no failure. The value of electric current flowing between theelectrodes at the reading timing is accordingly higher in the case wherethe particulate filter has a failure than in the case where theparticulate filter has no failure. This allows for diagnosis of whetherthe particulate filter has a failure by comparing the output value ofthe PM sensor at the reading timing with a predefined reference value.For example, with regard to the PM sensor that is configured to outputthe value of electric current flowing between the electrodes, when theoutput value of the PM sensor at the reading timing is lower than apredefined reference value (current value), diagnosis indicates that theparticulate filter has no failure. When the output value of the PMsensor at the reading timing is equal to or higher than the predefinedreference value, on the other hand, diagnosis indicates that theparticulate filter has a failure. With regard to the PM sensor that isconfigured to output the resistance value between the electrodes, whenthe output value of the PM sensor at the reading timing is higher than apredefined reference value (resistance value), diagnosis indicates thatthe particulate filter has no failure. When the output value of the Pmsensor at the reading timing is equal to or lower than the predefinedreference value, on the other hand, diagnosis indicates that theparticulate filter has a failure.

The predetermined time period herein denotes a time duration specifiedto provide a significant difference between the output values of the PMsensor at the reading timing in the case where the particulate filterhas a failure and in the case where the particulate filter has nofailure and may be determined in advance by a fitting operation based onan experiment or the like. The predefined reference value is providedsuch as to determine that at least part of the particulate filter has afailure such as breakage or erosion when the current value output fromthe PM sensor at the reading timing is equal to or higher than thepredefined reference value (or when the resistance value output from thePM sensor at the reading timing is equal to or lower than the predefinedreference value). In other words, the predefined reference valuecorresponds to a value output from the PM sensor at the reading timingwhen the measurement process is performed for a particulate filter thatis on the boundary between normal and abnormal.

Intensive experiments and examinations have resulted in obtaining thefinding that the output value of the PM sensor at the reading timing islower in the case where in-cylinder rich control is performed in thepredetermined time period than in the case where the in-cylinder richcontrol is not performed. This may be attributed to the followingreasons. The stronger linkage of SOF (Soluble Organic Fraction) withsoot included in the exhaust gas of the internal combustion engine isexpected in the case where the in-cylinder rich control is performedthan in the case where the in-cylinder rich control is not performed.Accordingly, when the in-cylinder rich control is not performed, SOF isexpected to hardly adhere and deposit between the electrodes of the PMsensor, while only soot is expected to adhere and deposit between theelectrodes of the PM sensor. When the in-cylinder rich control isperformed, on the other hand, SOF as well as soot is expected to adhereand deposit between the electrodes of the PM sensor. The electricalconductivity of SOF is lower than the electrical conductivity of soot.It is accordingly expected to increase the resistance value between theelectrodes and thereby decrease the value of electric current flowingbetween the electrodes in the case where a large amount of SOF depositsbetween the electrode of the PM sensor, compared with the case where asmall amount of SOF deposits, in the case where the in-cylinder richcontrol is performed in the predetermined time period, there is apossibility that the current value output from the PM sensor at thereading timing becomes lower than the predefined reference value (or theresistance value output from the PM sensor at the reading timing becomeshigher than the predefined reference value) even when the particulatefilter has a failure. When diagnosis of a failure of the particulatefilter is performed based on the output value of the PM sensor at thereading timing in the case where the in-cylinder rich control isperformed in the predetermined time period, there is accordingly apossibility that the particulate filter that actually has a failure isincorrectly diagnosed to have no failure.

The failure diagnosis device of the emission control system according tothe above aspect of the disclosure performs the sensor recovery processduring the in-cylinder rich control or triggered by termination of thein-cylinder rich control and subsequently performs the measurementprocess after termination of the, in-cylinder rich control and thesensor recovery process. This configuration enables PM depositingbetween the electrodes to he oxidized and removed during the in-cylinderrich control or at an earlier time after termination of the in-cylinderrich control. As a result, this enables the measurement process to bestarted at an earlier time after termination of the in-cylinder richcontrol. Starting the measurement process at the earlier time aftertermination of the in-cylinder rich control enables the measurementprocess to be terminated prior to next in-cylinder rich control. Thefailure diagnosis device of this aspect accordingly allows themeasurement process to be performed, while preventing the in-cylinderrich control from being performed in the predetermined time period. Whenthe sensor recovery process is triggered by termination of thein-cylinder rich control, the controller may perform preheat treatmentthat controls the heater to heat the sensor element to a specifiedtemperature lower than the temperature that allows for oxidation of PMduring the in-cylinder rich control. The “specified temperature” hereindenotes a temperature that is higher than a minimum temperature thatactivates the sensor element. There is a need to heat the sensor elementto the temperature that allows for oxidation of PM in the sensorrecovery process after termination of the in-cylinder rich control. Theconfiguration of performing the preheat treatment has preheated thesensor element and thus enables the temperature of the sensor element tobe quickly increased to the temperature that allows for oxidation of PM.The measurement process is started after oxidation and removal of PMdepositing between the electrodes. The failure diagnosis device of thisaspect allows the measurement process to be performed, while preventingnext in-cylinder rich control from being performed in the predeterminedtime period.

In the failure diagnosis device of the emission control system of theabove aspect, further comprising: that the controller configured to:predict whether the in-cylinder rich control is performed in thepredetermined time period, before the sensor recovery process isactually performed. When it is predicted that the in-cylinder richcontrol is performed in the predetermined time period, the controllermay perform the sensor recovery process during the in-cylinder richcontrol or triggered by termination of the in-cylinder rich control andsubsequently perform the measurement process after termination of thein-cylinder rich control and the sensor recovery process.

When it is predicted that the in-cylinder rich control is not performedin the predetermined time period, the controller may perform the sensorrecovery process and subsequently perform the measurement process aftertermination of the sensor recovery process.

On prediction that the in-cylinder rich control is performed in thepredetermined time period, the failure diagnosis device of this aspectperforms the sensor recovery process during the in-cylinder rich controlor triggered by termination of the in-cylinder rich control andsubsequently performs the measurement process after termination of thein-cylinder rich control and the sensor recovery process. Thisconfiguration accordingly enables the measurement process to beterminated prior to next in-cylinder rich control. On prediction thatthe in-cylinder rich control is not performed in the predetermined timeperiod, on the other hand, the failure diagnosis device of this aspectimmediately performs the sensor recovery process and subsequentlyperforms the measurement process after termination of the sensorrecovery process. This allows for quick detection of a failure of theparticulate filter.

In the failure diagnosis device of the emission control system of theabove aspect, the exhaust gas purification device may include an NO_(X)storage reduction catalyst (NSR catalyst) that is configured to storeNO_(X) in the exhaust gas when an air-fuel ratio of the exhaust gas is alean air-fuel ratio higher than the stoichiometric air-fuel ratio and toreduce NO_(X) stored in the NSR catalyst when the air-fuel ratio of theexhaust gas is a rich air-fuel ratio lower than the stoichiometricair-fuel ratio. The supplier may perform the in-cylinder rich controlfor the purpose of reducing NO_(X) stored in the NSR catalyst, when aNO_(X) storage amount of the NSR catalyst becomes equal to or greaterthan a predetermined upper limit storage amount. The controller maypredict that the in-cylinder rich control is performed in thepredetermined time period when the NO_(X) storage amount of the NSRcatalyst is equal to or greater than an allowable storage amount whichis smaller than the upper limit storage amount, while predicting thatthe in-cylinder rich control is not performed in the predetermined timeperiod. when the NO_(X) storage amount of the NSR catalyst is less thanthe allowable storage amount. The “allowable storage amount” herein isprovided as a NO_(X) storage amount that causes the in-cylinder richcontrol for the purpose of reducing NO_(X) stored in the NSR catalyst tobe performed in the predetermined time period in the case where themeasurement process is started in the state that the NO_(X) storageamount is equal to or greater than the allowable storage amount or as aNO_(X) storage amount that causes the in-cylinder rich control for thepurpose of reducing NO_(X) stored in the NSR catalyst to be performed inthe predetermined time period in the case where the NO_(X) storageamount becomes equal to or greater than the allowable storage amountduring the measurement process, and is determined in advance byexperiment. This configuration allows for prediction of whether thein-cylinder rich control is performed in the predetermined time period,before the in-cylinder rich control is actually performed for thepurpose of reducing NO_(X) stored in the NSR catalyst.

In the case where the exhaust gas purification device includes the NSRcatalyst, when the sulfur poisoning amount of the NSR catalyst isincreased to a certain level, in-cylinder rich control is performed forthe purpose of eliminating sulfur poisoning of the NSR catalyst. In theemission control system where the exhaust gas purification deviceincludes the NSR catalyst and the supplier performs the in-cylinder richcontrol when the sulfur poisoning amount of the NSR catalyst becomesequal to or greater than a predetermined upper limit poisoning amount,the controller may predict that the in-cylinder rich control isperformed in the predetermined time period when the sulfur poisoningamount of the NSR catalyst is equal to or greater than an allowablepoisoning amount which is smaller than the upper limit poisoning amount.The “allowable poisoning amount” herein is provided as a sulfurpoisoning amount that causes the in-cylinder rich control for thepurpose of eliminating sulfur poisoning of the NSR catalyst to beperformed in the predetermined time period in the case where themeasurement process is started in the state that the sulfur poisoningamount is equal to or greater than the allowable poisoning amount or asa sulfur poisoning amount that causes the in-cylinder rich control forthe purpose of eliminating sulfur poisoning of the NSR catalyst to beperformed in the predetermined time period in the case where the sulfurpoisoning amount becomes equal to or greater than the allowablepoisoning amount during the measurement process, and is determined inadvance by experiment. This configuration allows for prediction ofwhether the in-cylinder rich control is performed in the predeterminedtime period, before the in-cylinder rich control is actually performedfor the purpose of eliminating sulfur poisoning of the NSR catalyst.

In the failure diagnosis device of the emission control system of theabove aspect, the exhaust gas purification device may include aselective catalytic reduction catalyst (SCR catalyst) that is configuredto adsorb NH₃ included in the exhaust gas and reduce NO_(X) in theexhaust. gas using the adsorbed NH₃ as a reducing agent, and an NH₃producing catalyst that is placed upstream of the SCR catalyst and isconfigured to produce NH₃ when an air-fuel ratio of the exhaust gas is arich air-fuel ratio lower than the stoichiometric air-fuel ratio. Thesupplier may perform the in-cylinder rich control to produce NH₃ by theNH₃ producing catalyst when an NH₃ adsorption amount of the SCR catalystbecomes equal to or less than a predetermined lower limit adsorptionamount. The controller may predict that the in-cylinder rich control isperformed in the predetermined time period when the NH₃ adsorptionamount of the SCR catalyst is equal to or less than an allowableadsorption amount which is larger than the lower limit adsorptionamount, while predicting that the in-cylinder rich control is notperformed in the predetermined time period when the NH₃ adsorptionamount of the SCR catalyst is greater than the allowable adsorptionamount. The “allowable adsorption amount” herein is provided as an NH₃adsorption amount that causes the in-cylinder rich control for thepurpose of producing NH₃ to be performed in the predetermined timeperiod in the case where the measurement process is started in the statethat the NH₃ adsorption amount is equal to or less than the allowableadsorption amount or as an NH₃ adsorption amount that causes thein-cylinder rich control for the purpose of producing NH₃ to heperformed in the predetermined time period in the case where the NH₃adsorption amount becomes equal to or less than the allowable adsorptionamount during the measurement process, and is determined in advance byexperiment. This configuration allows for prediction of whether thein-cylinder rich control is performed in the predetermined time period,before the in-cylinder rich control is actually performed for thepurpose of producing NH₃.

In the failure diagnosis device of the emission control system thatutilizes the electrode-based PM sensor provided downstream of theparticulate filter in the exhaust conduit to diagnose a failure of theparticulate filter, the above aspects of the disclosed embodiments maysuppress reduction of the accuracy of diagnosis of a failure due to thein-cylinder rich control.

Further features of the disclosed embodiments will become apparent fromthe following description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the schematic configuration of aninternal combustion engine and its air intake and exhaust system withwhich an embodiment, referred to below as Embodiment 1, may be applied;

FIG. 2 is a diagram schematically illustrating the configuration of a PMsensor;

FIG. 3 is diagrams showing variations in output value of a PM sensorafter termination of a sensor recovery process;

FIG. 4 is diagrams showing variations in output value of the PM sensorafter termination of the sensor recovery process in the case where aparticulate filter has a failure;

FIG. 5 is a timing chart showing the execution timings of in-cylinderrich control for the purpose of reducing NO_(X) stored in an NSRcatalyst and the execution timings of the sensor recovery process;

FIG. 6 is a flowchart showing a processing routine performed by an ECUto diagnose a failure of the particulate filter according to Embodiment1;

FIG. 7 is a timing chart showing the execution timings of in-cylinderrich control for the purpose of eliminating sulfur poisoning of the NSRcatalyst and the execution timings of the sensor recovery process;

FIG. 8 is a diagram illustrating the schematic configuration of aninternal combustion engine and its air intake and exhaust system withwhich an embodiment, referred to below as Modification 2 of Embodiment1, may be applied;

FIG. 9 is a timing chart showing the execution timings of in-cylinderrich control for the purpose of supplying NH₃ to an SCR catalyst;

FIG. 10 is a flowchart showing a processing routine performed by the ECUto diagnose a failure of the particulate filter according to anembodiment referred to below as Embodiment 2;

FIG. 11 is a flowchart showing a processing routine performed by the ECUto diagnose a failure of the particulate filter according to anembodiment referred to below as Modification 1 of Embodiment 2; and

FIG. 12 is a flowchart showing a processing routine performed by the ECUto diagnose a failure of the particulate filter according to anembodiment referred to below as Modification 2 of Embodiment 2.

DESCRIPTION OF THE EMBODIMENTS

The following describes concrete embodiments of the disclosure withreference to the drawings. The dimensions, the materials, the shapes,the positional relationships and the like of the respective componentsdescribed in the following embodiments are only for the purpose ofillustration and not intended at all to limit the scope of theembodiments to such specific descriptions.

Embodiment 1

The following describes Embodiment 1 with reference to FIGS. 1 to 7.FIG. 1 is a diagram illustrating the configuration of an internalcombustion engine 1 and its air intake and exhaust system with whichdisclosed embodiments may be applied. The internal combustion engine 1shown in FIG. 1 is a compression ignition internal combustion engine(diesel engine) using light oil as fuel. The internal combustion engine1 may alternatively be a spark ignition internal combustion engine usinggasoline or the like as fuel, as long as it is operable using anair-fuel mixture having a lean air-fuel ratio that is higher than thestoichiometric air-fuel ratio.

The internal combustion engine 1 includes a fuel injection valve 3 thatis configured to inject the fuel into a cylinder 2. In the case of theinternal combustion engine 1 provided as the spark ignition internalcombustion engine, the fuel injection valve 3 may be configured toinject the fuel into an air intake port.

The internal combustion engine 1 is connected with an air intake pipe 4.An air flow meter 40 is placed in the middle of the air intake pipe 4 tooutput an electric signal reflecting the amount (mass) of the intake air(the air) flowing in the air intake pipe 4. An air intake throttle valve41 is placed downstream of the air flow meter 40 in the air intake pipe4 to change the passage cross-sectional area of the air intake pipe 4and thereby regulate the amount of the air taken into the internalcombustion engine 1.

The internal combustion engine 1 is connected with an exhaust pipe 5. Acatalyst casing SO is placed in the middle of the exhaust pipe 5. Thecatalyst casing 50 includes a cylindrical casing filled with an NSRcatalyst.

The NSR catalyst chemically absorbs or physically adsorbs NO_(X)included in the exhaust gas when the exhaust gas has a lean air-fuelratio higher than the stoichiometric air-fuel ratio, while releasingNO_(X) to accelerate reaction of the released NO_(X) with a reducingcomponent (for example, a hydrocarbon MO or carbon monoxide (CO)) whenthe exhaust gas has a rich air-fuel ratio lower than the stoichiometricair-fuel ratio, The catalyst casing 50 corresponds to one aspect of the“exhaust gas purification device” of the disclosure. An air-fuel ratiosensor 52 is mounted to the exhaust pipe 5 at a position upstream of thecatalyst casing 50 to output an electric signal related to the air-fuelratio of the exhaust gas flowing in the exhaust pipe 5.

A filter casing 51 is placed, downstream of the catalyst casing 50 inthe exhaust pipe 5. The filter casing 51 has a particulate filter placedinside of a cylindrical casing. The particulate filter is a wall-flowfilter made of a porous base material and serves to trap PM included inthe exhaust gas. An exhaust temperature sensor 53 and a PM sensor 54 areplaced downstream of the filter casing 51 in the exhaust pipe 5 tooutput an electric signal related to the temperature of the exhaust gasflowing in the exhaust pipe 5 and output an electric signal related tothe PM concentration of the exhaust gas flowing in the exhaust pipe 5,respectively.

The following describes the configuration of the PM sensor 54 withreference to FIG. 2. FIG. 2 is a diagram schematically illustrating theconfiguration of the PM sensor 54. The PM sensor 54 shown in FIG. 2 isan electrode-based PM sensor. Although the PM sensor 54 shown in FIG. 2includes a pair of electrodes, the PM sensor may include multiple pairsof electrodes. The PM sensor 54 has a sensor element 543 including apair of electrodes 541 and 542 that are placed away from each other on asurface of a plate-like insulator 540, an ammeter 544 that is configuredto measure the electric current flowing between the electrodes 541 and542, an electric heater 545 that is placed on a rear face of the sensorelement 543, and a cover 546 that is configured to cover the sensorelement 543. The cover 546 has a plurality of through holes 547 formedtherein.

In the state that the PM sensor 54 having the above configuration ismounted to the exhaust pipe 5, part of the exhaust gas flowing in theexhaust pipe 5 passes through the through holes 547 to flow in the cover546. When the exhaust gas flows in the cover 546, PM included in theexhaust gas adheres between the electrodes 541 and 542 (i.e., on theinsulator 540). PM has electrical conductivity, so that deposition of apredetermined amount of PM between the electrodes 541 and 542establishes electrical continuity between the electrodes 541 and 542.Applying a predetermined voltage from a power source to the electrodes541 and 542 causes the electric current to flow between the electrodes541 and 542 when the electrodes 541 and 542 are electrically whenelectrical continuity is established between the electrodes 541 and 549.

After the amount of PM depositing between the electrodes 541 and 542reaches the predetermined amount, the resistance value between theelectrodes 541 and 542 decreases with a further increase in amount of PMdepositing between the electrodes 541 and 542. Accordingly the value ofcurrent flowing between the electrodes 541 and 542 increases with anincrease in amount of PM depositing between the electrodes 541 and 542.The amount of PM depositing between the electrodes 541 and 542 is thusdetectable by measuring the value of current flowing between theelectrodes 541 and 542 with the ammeter 544.

When the amount of PM depositing between the electrodes 541 and 542 isless than the predetermined amount, the electrodes 541 and 542 have noelectrical continuity, so that the output of the PM sensor 54 is zero.When the amount of PM depositing between the electrodes 541 and 542reaches or exceeds the predetermined amount, the electrodes 541 and 542have electrical continuity so that the output of the PM sensor 54becomes higher than zero. After the electrical continuity is establishedbetween the electrodes 541 and 542, the output of the PM sensor 54increases with a further increase in amount of PM depositing between theelectrodes 541 and 542. In the description below, the predeterminedamount is referred to as “effective deposition amount”.

There is a limited space between the electrodes 541 and 542 to allow fordeposition of PM. When the amount of PM depositing between theelectrodes 541 and 542 (hereinafter referred to as “deposition amount ofPM”) reaches a predetermined upper limit amount (upper limit depositionamount), the electric current is supplied to the heater 545 to increasethe temperature of the sensor element 543 to a temperature that allowsfor oxidation of PM and oxidize and remove the PM depositing between theelectrodes 541 and 542 (sensor recovery process).

Referring back to FIG. 1, the internal combustion engine 1 is providedwith an ECU (electronic control unit) 6 as the controller of thedisclosure. The ECU 6 may be programmed to perform disclosed functionsThe ECU 6 is provided as an electronic control unit including a CPU, aROM, a RAM and a backup RAM. The ECU 6 is electrically connected withvarious sensors including an accelerator position sensor 7 and a crankposition sensor 8, in addition to the air flow meter 40, the air-fuelratio sensor 52, the exhaust temperature sensor 53 and the PM sensor 54described above. The accelerator position sensor 7 is provided as asensor that outputs an electric signal related to the operation amount(accelerator position) of an accelerator pedal (not shown). The crankposition sensor 8 is provided as a sensor that outputs an electricsignal related to the rotational position of an output shaft(crankshaft) of the internal combustion engine 1.

The ECU 6 is also electrically connected with various devices such asthe fuel injection valve 3 and the intake air throttle valve 41described above. The ECU 6 controls the above various devices, based onoutput signals from the above various sensors. For example, under acondition that the internal combustion engine 1 is operated withcombustion of the air-fuel mixture having a lean air-fuel ratio (leancombustion operation), when the NO_(X) storage amount of the NSRcatalyst reaches or exceeds a predefined upper limit storage amount, theECU 6 controls the fuel injection valve 3 to change the air-fuel ratioof the air-fuel mixture subjected to combustion in the cylinder 2 fromthe lean air-fuel ratio to a rich air-fuel ratio (in-cylinder richcontrol), so as to reduce and convert NO_(X) stored in the NSR catalyst(rich spike process). The ECU 6 performing the in-cylinder rich controlwhen the NO_(X) storage amount of the NSR catalyst becomes equal to orhigher than the upper storage amount implements the “supplier” of thedisclosure. The ECU 6 performs a failure diagnosis process of theparticulate filter which is characteristic of disclosed embodiments, inaddition to known processes such as the rich spike process describedabove. The following describes a procedure of the failure diagnosisprocess of the particulate filter.

The ECU 6 performs the sensor recovery process to remove the PMdepositing between the electrodes 541 and 542 of the PM sensor 54,before performing failure diagnosis of the particulate filter. Morespecifically, the ECU 6 causes electric current to be supplied from thepower source to the heater 545 of the PM sensor 54. Supplying electriccurrent to the heater 545 causes the heater 545 to generate heat andthereby heat the sensor element 543. The ECU 6 controls the value ofdriving current of the heater 545 to adjust the temperature of thesensor element 543 to a temperature that allows for oxidation of PM. Thetemperature of the sensor element 543 may be regarded as beingapproximately equal to the temperature of the heater 545. The ECU 6accordingly controls the current value to adjust the temperature of theheater 545 to the temperature that allows for oxidation of PM. Thetemperature of the heater 545 may be calculated from the resistancevalue of the heater 545.

When the state that the temperature of the sensor element 543 isadjusted to the temperature that allows for oxidation of PM continuesfor a specified recovery time, the ECU 6 stops the supply of electriccurrent to the heater 545 and terminates the sensor recovery process.The specified recovery time denotes a time duration required foroxidation and removal of the PM depositing between the electrodes 541and 542 of the PM sensor 54. For example, the specified recovery timemay he fixed to a time duration required for oxidation and removal ofthe predetermined upper limit deposition amount of PM or may be changedaccording to the actual deposition amount of PM.

After termination of the sensor recovery process, the ECU 6 startsapplication of the predetermined voltage to the electrodes 541 and 542of the PM sensor 54. The ECU 6 then reads an output value of the PMsensor 54 at a time when a predetermined time period has elapsed sincethe start of application of the predetermined voltage (reading timing)and compares the output value with a predefined reference value in orderto diagnose a failure of the particulate filter. The process of startingapplication of the predetermined voltage to the electrodes 541 and 542after termination of the sensor recovery process and subsequentlyreading the output value of the PM sensor 54 after elapse of thepredetermined time period corresponds to the “measurement process” ofthe disclosure.

When a failure such as breakage or erosion occurs in part of theparticulate filter, the PM trapping efficiency of the particulate filterdecreases. The amount of PM slipping through the particulate filter perunit time is larger in the case where the particulate filter has afailure than in the case where the particulate filter has no failure.

FIG. 3 is diagrams showing variations in deposition amount of PM in thePM sensor 54 and in output value of the PM sensor 54 after terminationof the sensor recovery process. FIG. 3(a) shows the elapsed time sincethe start of application of the predetermined voltage to the electrodes541 and 542 as abscissa and the deposition amount of PM in the PM sensor54 as ordinate. FIG. 3(b) shows the elapsed time since the, start ofapplication of the predetermined voltage to the electrodes 541 and 542as abscissa and the output value of the PM sensor 54 as ordinate.Solid-line graphs in FIGS. 3(a) and 3(b) show the deposition amount ofPM in the PM sensor 54 and the output value of the PM sensor 54 in thecase where the particulate filter has no failure. Dot-and-dash-linegraphs in FIGS. 3(a) and 3(b) show the deposition amount of PM in the PMsensor 54 and the output value of the PM sensor 54 in the case wherepart of the particulate filter has a failure. The solid-line graphs andthe dot-and-dash-line graphs show the results measured under identicalconditions other than the presence or the absence of a failure in theparticulate filter.

As shown in FIG. 3, immediately after a start of application of thepredetermined voltage to the electrodes 541 and 542, in both the casewhere the particulate filter has a failure and the case where theparticulate filter has no failure, the deposition amount of PM in the PMsensor 54 is less than the effective deposition amount, so that theoutput value of the PM sensor 54 is zero. The amount of PM slippingthrough the particulate filter per unit time is, however, larger in thecase where the particulate filter has a failure than in the case wherethe particulate filter has no failure. Accordingly, the timing when thedeposition amount of PM in the PM sensor 54 reaches the effectivedeposition amount is earlier in the case where the particulate filterhas a failure than in the case where the particulate filter has nofailure. As a result, the timing when the output value of the PM sensor54 starts increasing from zero (hereinafter referred to as “outputstart, timing”) is earlier in the case where the particulate filter hasa failure (t1 in FIG. 3) than in the case where the particulate filterhas no failure (t2 in FIG. 3). Additionally, the increase rate (increaseamount per unit time) of the output value after the output start timingis higher in the case where the particulate filter has a failure than inthe case where the particulate filter has no failure.

Here attention is focused on a predetermined timing (ts in FIG. 3) thatis later than the output start timing t1 in the case where theparticulate filter has a failure but is earlier than the output starttiming t2 in the case where the particulate filter has no failure. Atthis predetermined timing ts, the output value of the PM sensor is zeroin the case where the particulate filter has no failure, while beingequal to or higher than a predefined reference value (Tr in FIG. 3) thatis larger than zero in the case where the particulate filter has afailure.

By taking into account the above characteristic, the predetermined timeperiod may be set to make the reading timing equal to the predeterminedtiming ts. This allows for diagnosis of a failure of the particulatefilter by comparing the output value of the PM sensor 54 at the timewhen the predetermined time period has elapsed since termination of thesensor recovery process with the predefined reference value Tr.

The predetermined time period denotes a required time duration betweenthe time when application of the predetermined voltage to the electrodes541 and 542 is started and the time when the deposition amount of PM inthe PM sensor 54 becomes equal to or higher than the predefinedreference value Pr on the assumption that the particulate filter has afailure. The ECU 6 accordingly assumes that the particulate filter has afailure at the time when application of the predetermined voltage to theelectrodes 541 and 542 is started, and starts estimation (computation)of the amount of PM adhering to or depositing in the PM sensor 54. TheECU 6 determines that the predetermined time period has elapsed when theestimated deposition amount of PM reaches a predetermined depositionamount (for example, a deposition amount of PM that makes the outputvalue of the PM sensor 54 equal to or higher than the predefinedreference value Tr when part of the particulate filter has a failure).In the case where the output value of the PM sensor 54 is lower than thepredefined reference value Tr at the time when the predetermined timeperiod has elapsed (reading timing ts), the ECU 6 diagnoses that theparticulate filter has no failure, in the case where the output value ofthe PM sensor 54 is equal to or higher than the predefined referencevalue Tr at the reading timing ts, on the other hand, the CPU 6diagnoses that the particulate filter has a failure.

The output value of the PM sensor 54 is likely to include a measurementerror, due to an initial tolerance of the PM sensor 54 or the like. Theestimated deposition amount of PM is likely to include an estimationerror, it is accordingly desirable to set the reading timing(predetermined timing) ts and the predefined reference value Trrespectively to a timing and a value that ensure diagnosis of a failurewith high accuracy even when the measurement value of the PM sensor 54includes a measurement error and the estimated deposition amount of PMincludes an estimation error. For example, it is desirable to set thepredefined reference value Tr to a sufficiently large value relative tothe measurement error of the PM sensor 54 and the estimation error ofthe estimated deposition amount of PM and to set the reading timing isaccording to the setting of the predefined reference value Tr.

The estimated deposition amount of PM is estimated by integrating theamount of PM depositing in the PM sensor 54 per unit time on theassumption that the particulate filter has a failure. The amount of PMdepositing in the PM sensor 54 per unit time is related to the flow rateof the exhaust gas (flow velocity of the exhaust gas)) flowing out ofthe particulate filter per unit time, the PM concentration of theexhaust gas flowing out of the particulate filter having a failure, andthe difference between the temperature of the exhaust gas flowing out ofthe particulate filter and the temperature of the sensor element 543.For example, the amount of PM depositing in the PM sensor 54 per unittime increases with a decrease in flow rate of the exhaust gas. Theamount of PM depositing in the PM sensor 54 per unit time also increaseswith an increase in PM concentration of the exhaust gas flowing out ofthe particulate filter having a failure. Additionally, the amount of PMdepositing in the PM sensor 54 per unit time increases with an increasein difference between the temperature of the exhaust gas flowing out ofthe particulate filter and the temperature of the sensor element 543(i.e., with an increase in temperature of the exhaust gas relative tothe temperature of the sensor element 543). The amount of PM depositingin the PM sensor 54 per unit time may thus be estimated by using, asparameter, the flow rate of the exhaust gas flowing out of theparticulate filter per unit time, the PM concentration of the exhaustgas flowing out of the particulate filter on the assumption that theparticulate filter has a failure, and the difference between thetemperature of the exhaust gas flowing out of the particulate filter(i.e., output value of the exhaust temperature sensor 53) and thetemperature of the sensor element 543 (i.e., temperature calculated fromthe resistance value of the heater 545).

The flow rate of the exhaust gas flowing out of the particulate filterper unit time may be regarded as being equal to the amount of intake airper unit time and may this be determined by using the output value ofthe air flow meter 40 as a parameter. The PM concentration of theexhaust gas flowing out of the particulate filter having a failure maybe calculated by using, as parameters, the amount of PM discharged fromthe internal combustion engine 1 per unit time, the ratio of the amountof PM flowing out of the particulate filter having a failure to theamount of PM flowing into the particulate filter having the failure, andthe flow rate of the exhaust gas flowing out of the particulate filterper unit time. The PM adhering or depositing between the electrodes 541and 542 of the PM sensor 54 is mostly soot. It is accordingly preferableto determine the PM concentration of the exhaust gas flowing out of theparticulate filter having a failure by using, as parameters, the amountof soot discharged from the internal combustion engine 1 per unit time,the ratio of the amount of soot flowing out of the particulate filterhaving a failure to the amount of soot flowing into the particulatefilter having the failure (hereinafter referred to as “soot slip rate”)and the flow rate of the exhaust gas flowing out of the particulatefilter per unit time.

The amount of soot discharged from the internal combustion engine 1 perunit time is related to, for example, the amount of intake air, theamount of fuel injection, the temperature and the humidity and may thusbe determined by referring to a predetermined map or computation modelthat uses these relating factors as parameters. The soot slip rate ofthe particulate filter having a failure is related to the amount of PMtrapped by the particulate filter (hereinafter referred to as “trappedamount of PM”) and the flow rate of the exhaust gas flowing into theparticulate filter per unit time. For example, the soot slip rateincreases with an increase in trapped amount of PM by the particulatefilter. The soot slip rate also increases with an increase in flow rateof the exhaust gas flowing into the particulate filter per unit time.Accordingly, the soot slip rate of the particulate filter having afailure may be determined by referring to a predetermined map orcomputation model that uses, as parameters, the trapped amount of PM bythe particulate filter and the flow rate of the exhaust gas flowing intothe particulate filter per unit time. The trapped amount of PM by theparticulate filter may be calculated by using the operation history ofthe internal combustion engine 1 (for example, the integrated values ofthe amount of fuel injection and the amount of intake air) as parametersor may be calculated from the output value of a differential pressuresensor (not shown) that is configured to detect a pressure differencebefore and after the particulate filter.

Intensive experiments and examinations have resulted in obtaining thefinding that the output value of the PM sensor 54 at the reading timingts is lower in the case where the in-cylinder rich control describedabove is performed in a time period between the start of application ofthe predetermined voltage to the electrodes 541 and 542 and the readingtiming ts (i.e., in the predetermined time period) than in the casewhere the in-cylinder rich control is not performed.

FIG. 4 is diagrams showing variations in deposition amount of PM and inoutput value of the PM sensor 54 after termination of the sensorrecovery process in the case where the particulate filter has a failure.FIG. 4(a) shows the elapsed time since the start of application of thepredetermined voltage to the electrodes 541 and 542 as abscissa and thedeposition amount of PM in the PM sensor 54 as ordinate. FIG. 4(b) showsthe elapsed time since the start of application of the predeterminedvoltage to the electrodes 541 and 542 as abscissa and the output valueof the PM sensor 54 when the particulate filter has a failure asordinate. A solid-line graph in FIG. 4(b) shows the output value of thePM sensor 54 when the in-cylinder rich control is performed in at leastpart of the predetermined time period. A dot-and-dash-line graph in FIG.4(b) shows the output value of the PM sensor 54 when the in-cylinderrich control is not performed in the predetermined time period. Thesolid-line graph and the dot-and-dash-line graph in FIG. 4(b) show theresults measured under identical conditions other than execution ornon-execution of the in-cylinder rich control.

As shown in FIG. 4, the case where the in-cylinder rich control isperformed in the predetermined time period and the case where thein-cylinder rich control is not performed in the predetermined timeperiod have substantially similar output start timing of the PM sensor54 (t3 in FIG. 4) but have different output values of the PM sensor 54after the output start timing. More specifically, the output value ofthe PM sensor 54 is lower in the case where the in-cylinder rich controlis performed in the predetermined time period than in the case where thein-cylinder rich control is not performed in the predetermined timeperiod. Accordingly, the output value of the PM sensor 54 at the readingtiming ts is lower in the case where the in-cylinder rich control isperformed (Cpm2 in FIG. 4(b)) than in the case where the in-cylinderrich control is not performed (Cmp1 in FIG. 4(b)).

The mechanism of the phenomenon shown in FIG. 4 is not clearlyelucidated but is estimated as follows. The stronger linkage of SOF(soluble organic fraction) with soot included in the exhaust gas of theinternal combustion engine 1 is expected in the case where thein-cylinder rich control is performed than in the case where thein-cylinder rich control is not performed. Accordingly, when thein-cylinder rich control is not performed, SOF is expected to hardlyadhere and deposit between the electrodes 541 and 542 of the PM sensor54, while only soot is expected to adhere and deposit between theelectrodes 541 and 542 of the PM sensor 54. When the in-cylinder richcontrol is performed, on the other hand, SOF as well as soot is expectedto adhere and deposit between the electrodes 541 and 542 of the PMsensor 54. The electrical conductivity of SOF is lower than theelectrical conductivity of soot. It is accordingly expected to increasethe electric resistance between the electrodes 541 and 542 and therebydecrease the output value of the PM sensor 54 in the case where SOFdeposits between the electrodes 541 and 542 of the PM sensor 54,compared with the case where SOF hardly deposits. In other words, in thecase where PM including SOF deposits between the electrodes 541 and 542,the value of current flowing between the electrodes 541 and 542 isexpected to he lower than the current value corresponding to the actualdeposition amount of PM.

When the in-cylinder rich control is performed in at least part of thepredetermined time period, SOF deposits between the electrodes 541 and542 of the PM sensor 54. As shown by the solid-line graph in FIG. 4(b),the output value Cpm2 of the PM sensor 54 at the reading timing ts islikely to he lower than the predefined reference value Tr. This may leadto incorrect diagnosis that the particulate filter that actually has afailure is diagnosed to have no failure.

In order to avoid such incorrect diagnosis due to the deposition of SOFdescribed above, the procedure of this embodiment performs the sensorrecovery process and the measurement process according to the executiontiming of the in-cylinder rich control. More specifically, as shown inFIG. 5, at the time when in-cylinder rich control is activated (when an“in-cylinder rich control flag” is set ON in FIG. 5) by the NO_(X)storage amount of the NSR catalyst reaching or exceeding the upper limitstorage amount described. above, the ECU 6 activates the sensor recoveryprocess (sets a “sensor recovery process flag” ON in FIG. 5). The ECU 6then performs the measurement process after termination of thein-cylinder rich control and the sensor recovery process. Theconfiguration of performing the sensor recovery process and themeasurement process according to the execution timing of the in-cylinderrich control enables the measurement process to be started at an earliertime after termination of the in-cylinder rich control. In other words,this enables the measurement process to be started when the NO_(X)storage amount of the NSR catalyst is sufficiently smaller than theupper limit storage amount. As a result, this enables the measurementprocess to he completed prior to subsequent execution of the in-cylinderrich control and thereby prevents the in-cylinder rich control frombeing performed in the predetermined time period. This reduces theincorrect diagnosis that the particulate filter that actually has afailure is diagnosed to have no failure.

The following describes a procedure of failure diagnosis processaccording to this embodiment with reference to FIG. 6. FIG. 6 is aflowchart showing a processing routine performed by the ECU 6 todiagnose a failure of the particulate filter. This processing routine isstored in advance in the ROM of the ECU 6 and is repeatedly performed atpredetermined time intervals during operation of the internal combustionengine 1. This processing routine is performed on the premise that theinternal combustion engine 1 and the PM sensor 54 are operated normally.

In the processing routine of FIG. 6, the ECU 6 first determines whethera failure detection flag is equal to value “0” at S101. The failuredetection flag denotes a storage area provided in advance in, forexample, the backup RAM of the ECU 6 and is set to “0” upondetermination that the particulate filter is normal in this processingroutine, while being set to “1” upon determination that the particulatefilter has a failure in this processing routine. In the case of anegative answer at S101, the ECU 6 terminates this processing routine.In the case of an affirmative answer at S101, on the other hand, the ECU6 proceeds to S102.

At S102, the ECU 6 determines whether conditions of the measurementprocess (measurement conditions) are satisfied. More specifically, theECU 6 determines that the measurement conditions are satisfied uponsatisfaction of specified conditions, for example, that the measurementprocess is not performed at the current moment and that electric powerrequired for the sensor recovery process is obtainable (i.e., the stateof charge of a battery or the amount of power generation by a generatorexceeds the amount of electric power required for the sensor recoveryprocess). In the case of a negative answer at S102, the ECU 6 terminatesthis processing routine, In the case of an affirmative answer at S102,on the other hand, the ECU 6 proceeds to S103.

At step S103, the ECU 6 determines whether the in-cylinder rich controlflag is ON, i.e., whether the in-cylinder rich control is beingperformed. In the case of a negative answer at S103, the ECU 6terminates this processing routine. In the case of an affirmative answerat S103, on the other hand, the ECU 6 proceeds to S104.

At S104, the ECU 6 sets the sensor recovery process flag ON and suppliesthe electric current to the heater 545 of the PM sensor 54, so as tostart the sensor recovery process. At S105, the ECU 6 subsequentlydetermines whether the execution time of the sensor recovery process isequal to or longer than the specified recovery time. In the case of anegative answer at S105, the ECU 6 returns to S104 to continue thesensor recovery process. In the case of an affirmative answer at S105,on the other hand, the ECU 6 proceeds to S106 to stop the supply ofelectric current to the heater 545 and sets the sensor recovery processflag OFF, so as to terminate the sensor recovery process. Afterterminating the sensor recovery process at S106, the ECU 6 proceeds toS107.

At step S107, the ECU 6 determines whether the in-cylinder rich controlflag is OFF, i.e., whether the in-cylinder rich control has beenterminated. In the case of a negative answer at S107, this means thatthe in-cylinder rich control has not yet been terminated at the timewhen the sensor recovery process is terminated. The ECU 6 accordinglyrepeats the processing of S107. In the case of an affirmative answer atS107, on the other hand, this means that the in-cylinder rich controlhas already been terminated at the time when the sensor recovery processis terminated. The ECU 6 accordingly proceeds to S108.

At S108, the ECU 6 applies the predetermined voltage to the electrodes541 and 542 of the PM sensor 54 to start the measurement process.Immediately after termination of the sensor recovery process, the sensorelement 543 is in a high temperature atmosphere, so that there is apossibility that PM flowing in between the electrodes 541 and 542 doesnot deposit but is oxidized. Accordingly, it is desirable that, the ECU6 starts application of the predetermined voltage to the electrodes 541and 542 when the temperature of the sensor element 543 decreases to atemperature range where PM is not oxidized. When application of thepredetermined voltage to the electrodes 541 and 542 is startedimmediately after termination of the sensor recovery process, thepredetermined time period described later may be set to include a timeduration required for decreasing the temperature of the sensor element543 to the temperature range where PM is not oxidized.

At S109, the ECU 6 computes an elapsed time from the start ofapplication of the predetermined voltage to the electrodes 541 and 542to the current moment and determines whether the elapsed time is equalto or longer than the predetermined time period. As described above, thepredetermined time period denotes a required time duration between thetime when application of the predetermined voltage to the electrodes 541and 542 is started and the time when the deposition amount of PM in thePM sensor 54 becomes equal to or higher than the predefined referencevalue Tr on the assumption that the particulate filter has a failure. Inthe case of a negative answer at S109, the ECU 6 returns to S108. In thecase of an affirmative answer at S109, on the other hand, the ECU 6proceeds to S110.

At S110, the ECU 6 reads the output value of the PM sensor 54 (Cpm inFIG. 6). The output value Cpm read at S110 is the output value of the PMsensor 54 when the predetermined time period has elapsed since the startof application of the predetermined voltage to the electrodes 541 and542 and corresponds to the output value of the PM sensor 54 at thepredetermined timing (reading timing) is described above with referenceto FIG. 3.

On completion of the processing at S110, the ECU 6 diagnoses a failureof the particulate filter by the processing of S111 to S113. At S111,the ECU 6 determines whether the output value Cpm read at S110 is lowerthan the predefined reference value Tr, In the case of an affirmativeanswer at S111 (Cpm<Tr), the ECU 6 proceeds to S112 to determine thatthe particulate filter is normal (has no failure) and stores “0” in thefailure detection flag. in the case of a negative answer at S111(Cpm≧Tr), on the other hand, the ECU 6 proceeds to S113 to determinethat the particulate filter has a failure and stores “1” in the failuredetection flag. Upon determination that the particulate filter has afailure at S113, the ECU 6 may store failure information of theparticulate filter in the backup RAM or the like and may turn on amalfunction indication lamp (MIL) provided in the vehicle interior.

Performing the diagnosis of a failure of the particulate filteraccording to the processing routine of FIG. 6 as described above enablesthe measurement process to he performed at an earlier time aftertermination of the in-cylinder rich control and thereby prevents nextin-cylinder rich control from being performed in the predetermined timeperiod. This reduces the incorrect diagnosis that the particulate filterthat actually has a failure is diagnosed to have no failure.

The processing routine of FIG. 6 describes the configuration that thesensor recovery process is started at a certain timing during executionof the in-cylinder rich control. The sensor recovery process may,however, be started simultaneously with start of the in-cylinder richcontrol or may be started immediately before start of the in-cylinderrich control (for example, when the NO_(X) storage amount of the NSRcatalyst reaches a specified amount that is slightly smaller than theupper limit storage amount). Starting the sensor recovery process at anyof such timings enables the sensor recovery process to be terminatedduring execution of the in-cylinder rich control or at an earlier timeafter termination of the in rich control. This advances the executiontiming of the measurement process and thereby more effectively preventsnext in-cylinder rich control from being performed in the predeterminedtime period.

This embodiment describes the configuration that the sensor recoveryprocess is performed during execution of the in-cylinder rich control.According to a modification, the sensor recovery process and themeasurement process may he triggered by termination of the in-cylinderrich control and sequentially performed. This modified configurationalso enables the measurement process to be completed at an earlier timeafter termination of the in-cylinder rich control. In the configurationthat the sensor recovery process and the measurement process aretriggered by termination of the in-cylinder rich control and aresequentially performed, preheat treatment may be performed with theheater 545 during execution of the in-cylinder rich control to heat thesensor element 543 to a specified temperature that is lower than thetemperature that allows for oxidation of PM and higher than the minimumtemperature to activate the sensor element 543. In this modifiedconfiguration, the sensor element 543 is preheated at the time when thein-cylinder rich control is terminated. This shortens a required timeduration for heating the sensor element 543 to the temperature thatallows for oxidation of PM in the sensor recovery process. This resultsin terminating the sensor recovery process at an earlier time aftertermination of the in-cylinder rich control and starting the measurementprocess.

Modification 1 of Embodiment 1

In Embodiment 1 described above, the sensor recovery process and themeasurement process are performed according to the execution timing ofthe in-cylinder rich control for the purpose of reducing NO_(X) storedin the NSR catalyst. According to a modification, the sensor recoveryprocess and the measurement process may be performed according to theexecution timing of in-cylinder rich control for the purpose ofeliminating sulfur poisoning of the NSR catalyst.

The NSR catalyst placed in the catalyst casing 50 stores sulfurcompounds (SO_(X)) included in the exhaust gas, in addition to NOincluded in the exhaust gas. The NO_(X) storage capacity of the NSRcatalyst decreases with an increase in amount of SO_(X) stored in theNSR catalyst. There is accordingly a need to perform a process ofremoving SO_(X) stored in the NSR catalyst when the amount of SO_(X)stored in the NSR catalyst (sulfur poisoning amount) reaches or exceedsa predetermined upper limit poisoning amount. For removal of SO_(X)stored in the NSR catalyst, the NSR catalyst should be athigh-temperature and in a rich atmosphere. The ECU 6 accordinglyperforms in-cylinder rich control when the sulfur poisoning amount ofthe NSR catalyst reaches or exceeds the upper limit poisoning amount.This increases the temperature of the NSR catalyst with the heat ofoxidation reaction of the fuel included in the exhaust gas and controlsthe atmosphere of the NSR catalyst to the rich atmosphere. As in thecase where the in-cylinder rich control for the purpose of reducingNO_(X) stored in the NSR catalyst is performed in the predetermined timeperiod, performing such in-cylinder rich control in the predeterminedtime period may lead to inaccurate diagnosis of a failure of theparticulate filter.

As shown in FIG. 7, at the time when in-cylinder rich control isactivated (when an “in-cylinder rich control flag” is set ON in FIG. 7)by the sulfur poisoning amount of the NSR catalyst reaching or exceedingthe upper limit poisoning amount described above, the ECU 6 activatesthe sensor recovery process (sets a “sensor recovery process flag” ON inFIG. 7). The ECU 6 then performs the measurement process aftertermination of the in-cylinder rich control and the sensor recoveryprocess. The configuration of performing the sensor recovery process andthe measurement process according to the execution timing of thein-cylinder rich control enables the measurement process to he startedat an earlier time after termination of the in-cylinder rich control. Inother words, this enables the measurement process to be started when thesulfur poisoning amount of the NSR catalyst is sufficiently smaller thanthe upper limit poisoning amount. As a result, this prevents thein-cylinder rich control from being performed in the predetermined timeperiod and thus ensures accurate diagnosis of a failure of theparticulate filter.

The ECU 6 may perform the sensor recovery process and the measurementprocess according to the execution timing of in-cylinder rich controlfor the purpose of reducing NO_(X) stored in the NSR catalyst, additionto the execution timing of in-cylinder rich control for eliminatingsulfur poisoning of the NSR catalyst. This increases the opportunity ofperforming diagnosis of a failure of the particulate filter and therebyallows for earlier detection of a failure of the particulate filter.

Modification 2 of Embodiment 1

In Embodiment 1 described above, the sensor recovery process and themeasurement process are performed according to the execution timing ofthe in-cylinder rich control for the purpose of reducing NO_(X) storedin the NSR catalyst. According to a modification, the sensor recoveryprocess and the measurement process may be performed according to theexecution timing of in-cylinder rich control for the purpose ofsupplying NH₃ to an SCR catalyst placed downstream of the NSR catalyst.

FIG. 8 is a diagram illustrating the configuration of an internalcombustion engine and its air intake and exhaust system with whichdisclosed embodiments may be applied. The like components to those ofFIG. 1 described above are expressed by the like numerical symbols inFIG. 8. As shown in FIG. 8, a catalyst casing 55 is placed between thecatalyst casing 50 and the filter casing 51 in the exhaust pipe 5. Thiscatalyst casing 55 includes a cylindrical casing filled with the SCRcatalyst. The SCR catalyst serves to adsorb NH₃ included in the exhaustgas and reduce NO_(X) in the exhaust gas using the adsorbed NH₃ as areducing agent. In the emission control system having thisconfiguration, when the NH₃ adsorption amount of the SCR catalystdecreases to or below a predetermined lower limit adsorption amount, theECU 6 starts in-cylinder rich control for the purpose of supplying HN₃to the SCR catalyst. When the in-cylinder rich control is performed,NO_(X) stored in the NSR catalyst of the catalyst casing 50 is reducedto produce NH₃. The NSR catalyst of the catalyst casing 50 accordinglyworks as the NH₃ producing catalyst. NH₃ produced by the NSR catalystflows, along with the exhaust gas, into the catalyst casing 55 and isadsorbed to the SCR catalyst. As in the case where the in-cylinder richcontrol for the purpose of reducing NO_(X) stored in the NSR catalyst isperformed, performing the in-cylinder rich control for the purpose ofsupplying NH₃ to the SCR catalyst in the predetermined time period maylead to inaccurate diagnosis of a failure of the particulate filter.

As shown in FIG. 9, at the time when in-cylinder rich control isactivated (when an “in-cylinder rich control flag” is set ON in FIG. 9)by the NH₃ adsorption amount of the SCR catalyst decreasing to or belowthe lower limit adsorption amount described above, the ECU 6 activatesthe sensor recovery process (sets a “sensor recovery process flag” ON inFIG. 9). The ECU 6 then performs the measurement process aftertermination of the in-cylinder rich control and the sensor recoveryprocess. The configuration of performing the sensor recovery process andthe measurement process according to the execution timing of thein-cylinder rich control enables the measurement process to be startedat an earlier time after termination of the in-cylinder rich control. Inother words, this enables the measurement process to be started when theNH₃ adsorption amount of the SCR catalyst is sufficiently larger thanthe lower limit adsorption amount. As a result, this prevents thein-cylinder rich control from being performed in the predetermined timeperiod and thus ensures accurate diagnosis of a failure of theparticulate filter.

The failure diagnosis process described in this modification is alsoapplicable to the case where a three-way catalyst, instead of the NSRcatalyst, is placed in the catalyst casing 50. In this modifiedconfiguration, when the NH₃ adsorption amount of the SCR, catalystdecreases to or below the lower limit adsorption amount described above,in-cylinder rich control is performed for the purpose of producing NH₃by the three-way catalyst.

The ECU 6 may perform the sensor recovery process and the measurementprocess according to the execution timing of in-cylinder rich controlfor eliminating sulfur poisoning of the NSR catalyst and/or according tothe execution timing of in-cylinder rich control for the purpose ofreducing NO_(X) stored in the NSR catalyst, in addition to the executiontiming of in-cylinder rich control for the purpose of supplying NH₃ tothe SCR catalyst. This increases the opportunity of performing diagnosisof a failure of the particulate filter and thereby allows for earlierdetection of a failure of the particulate filter.

Embodiment 2

The following describes Embodiment 2 of the disclosure with reference toFIG. 10. The following describes only a configuration different fromEmbodiment 1 described above and does not specifically describe thesimilar configuration. The procedure of Embodiment 1 performs the sensorrecovery process and the measurement process according to the executiontiming of the in-cylinder rich control as described above. The procedureof Embodiment 2, on the other hand, predicts whether in-cylinder richcontrol is performed in the predetermined time period before the sensorrecovery process is actually started. On prediction that in-cylinderrich control is performed in the predetermined time period, like theprocedure of Embodiment 1, the procedure of Embodiment 2 performs thesensor recovery process and the measurement process according to theexecution timing of the in-cylinder rich control. On prediction thatin-cylinder rich control is not performed in the predetermined timeperiod, on the other hand, the procedure of Embodiment 2 performs thesensor recovery process and the measurement process immediately.

More specifically when the NO_(X) storage amount of the NSR catalyst isless than a predetermined NO_(X) storage amount (allowable storageamount) that is smaller than the upper limit storage amount describedabove, the ECU 6 predicts that in-cylinder rich control is not performedin the predetermined time period. When the NO_(X) storage amount of theNSR catalyst is equal to or greater than the allowable storage amount,on the other hand, the ECU 6 predicts that in-cylinder rich control isperformed in the predetermined time period. The “allowable storageamount” is provided as a NO_(X) storage amount that causes in-cylinderrich control for the purpose f reducing NO_(X) stored in the NSRcatalyst to he performed in the predetermined time period in the casewhere the sensor recovery process and the measurement process areperformed in the state that the NO_(X) storage amount of the NSRcatalyst is equal to or greater than the allowable storage amount, andmay be determined in advance by a fitting operation based on anexperiment or the like.

The following describes a procedure of failure diagnosis processaccording to this embodiment with reference to FIG. 10. FIG. 10 is aflowchart showing a processing routine performed by the ECU 6 todiagnose a failure of the particulate filter. The like steps to those ofthe processing routine of FIG. 6 are expressed by the like step numbersin. FIG. 10.

In the processing routine of FIG. 10, in the case of an affirmativeanswer at S102, the ECU 6 determines that there is a request forperforming the measurement process and proceeds to S201. At S201, theECU 6 predicts whether performing the sensor recovery process and themeasurement process at the current moment leads to the possibility thatin-cylinder rich control is performed in the predetermined time period.More specifically, the ECU 6 determines whether the NO_(X) storageamount of the NSR catalyst (Anox in FIG. 10) is less than the allowablestorage amount (Anoxt in FIG. 10). The NO_(X) storage amount Anox of theNSR catalyst is determined by a separate processing routine and isstored in a specified storage area of the backup RAM. The NO_(X) storageamount Anox may be estimated and computed based on the operation historyof the internal combustion engine 1 (for example, the amount of intakeair and the amount of fuel injection) or may be calculated from themeasurement values of NO_(X) sensors provided before and after thecatalyst casing 50.

In the case of an affirmative answer at S201, it is predicted that thein-cylinder rich control is not performed in the predetermined timeperiod. The ECU 6 accordingly proceeds to S202 to start the sensorrecovery process. When the execution time of the sensor recovery processbecomes equal to or longer than the specified recovery time, the ECU 6has an affirmative answer at S203 and proceeds to S204. At S204, the ECU6 stops the supply of electric current to the heater 545 to terminatethe sensor recovery process. After the processing of S204, the ECU 6performs the processing of and after S108.

In the case of a negative answer at S201, on the other hand, it ispredicted that the in-cylinder rich control is performed in thepredetermined time period. The ECU 6 accordingly proceeds to S205without starting the sensor recovery process. At S205, the ECU 6performs the sensor recovery process according to the execution timingof the in-cylinder rich control, like the procedure of Embodiment 1described above. More specifically, at S205, the ECU 6 performs thesensor recovery process according to the same procedure as that of S103to S107 in the processing routine of FIG. 6. After terminating thesensor recovery process, the ECU 6 performs the processing of and afterS108.

As described above, in the diagnosis of a failure of the particulatefilter by the processing routine of FIG. 10, on prediction that thein-cylinder rich control is performed in the predetermined time period,the sensor recovery process is performed according to the executiontiming of the in-cylinder rich control. This enables the measurementprocess to be performed at an earlier time after termination of thein-cylinder rich control and thereby prevents the in-cylinder richcontrol from being performed in the predetermined time period. Onprediction that the in-cylinder rich control is not performed in thepredetermined time period, on the other hand, the sensor recoveryprocess and the measurement process are performed immediately. Thisensures quick detection of a failure of the particulate filter.

Modification 1 of Embodiment 2

The procedure of Embodiment 2 described above predicts whether thein-cylinder rich control for the purpose of reducing NO_(X) stored inthe NSR catalyst is performed in the predetermined time period andcontrols the execution timings of the sensor recovery process and themeasurement process based on the prediction. A modification may predictwhether in-cylinder rich control for the purpose of eliminating sulfurpoisoning of the NSR catalyst is performed in the predetermined timeperiod and control the execution timings of the sensor recording processand the measurement process based on the prediction. More specifically,when the sulfur poisoning amount of the NSR catalyst is less than apredetermined sulfur poisoning amount (allowable poisoning amount) thatis smaller than the upper limit poisoning amount described above, theECU 6 predicts that in-cylinder rich control for the purpose ofeliminating sulfur poisoning of the NSR catalyst is not performed in thepredetermined time period. When the sulfur poisoning amount of the NSRcatalyst is equal to or greater than the allowable poisoning amount, onthe other hand, the ECU 6 predicts that in-cylinder rich control for thepurpose of eliminating sulfur poisoning of the NSR catalyst is performedin the predetermined time period. The “allowable poisoning amount” isprovided as a sulfur poisoning amount that causes in-cylinder richcontrol for the purpose of eliminating sulfur poisoning of the NSRcatalyst to he performed in the predetermined time period in the casewhere the sensor recovery process and the measurement process areperformed in the state that the sulfur poisoning amount of the NSRcatalyst is equal to or greater than the allowable poisoning amount, andmay he determined in advance by a fitting operation based on anexperiment or the like.

The following describes a procedure of failure diagnosis processaccording to this modification with reference to FIG. 11. FIG. 11 is aflowchart showing a processing routine performed by the ECU 6 todiagnose a failure of the particulate filter. The like steps to those ofthe processing routine of FIG. 10 are expressed by the like step numbersin FIG. 11.

In the processing routine of FIG. 11, in the case of an affirmativeanswer at S102, the ECU 6 proceeds to S301, instead of S201. At S301,the ECU 6 determines whether the sulfur poisoning amount of the NSRcatalyst (Asox in FIG. 11) is less than the allowable poisoning amount(Asoxt in FIG. 11). The sulfur poisoning amount of the NSR catalyst mayhe estimated based on the operation history of the internal combustionengine 1 (for example, the integrated values of the amount of fuelinjection and the amount of intake air) or may be calculated from themeasurement value of a SO_(X) sensor provided upstream of the catalystcasing 50.

In the case of an affirmative answer at S301 it is predicted that thein-cylinder rich control for the purpose of eliminating sulfur poisoningof the NSR, catalyst is not performed in the predetermined time period.The ECU 6 accordingly proceeds to S202 to start the sensor recoveryprocess. In the case of a negative answer at S301, on the other hand, itis predicted that the in-cylinder rich control for the purpose ofeliminating sulfur poisoning of the NSR catalyst is performed in thepredetermined time period. The ECU 6 accordingly proceeds to S205 toperform the sensor recovery process according to the execution timing ofthe in-cylinder rich control.

As described above, in the diagnosis of a failure of the particulatefilter by the processing routine of FIG. 11, on prediction that thein-cylinder rich control for the purpose of eliminating sulfur poisoningof the NSR catalyst is performed in the predetermined time period, thesensor recovery process is performed according to the execution timingof the in-cylinder rich control. This enables the measurement process tobe performed at an earlier time after termination of the in-cylinderrich control and thereby prevents the in-cylinder rich control frombeing performed in the predetermined time period. On prediction that thein-cylinder rich control for the purpose of eliminating sulfur poisoningof the NSR catalyst is not performed in the predetermined time period,on the other hand, the sensor recovery process and the measurementprocess are performed immediately. This ensures quick detection of afailure of the particulate filter.

Upon satisfaction of at least one of the conditions that the sulfurpoisoning amount of the NSR catalyst is equal to or greater than theallowable poisoning amount and that the NO_(X) storage amount of the NSRcatalyst is equal to or greater than the allowable storage amount, theECU 6 may predict that the in-cylinder rich control is performed in thepredetermined time period.

Modification 2 of Embodiment 2

The procedure of Embodiment 2 described above predicts whether thein-cylinder rich control for the purpose of reducing NO_(X) stored inthe NSR catalyst is performed in the predetermined time period andcontrols the execution timings of the sensor recovery process and themeasurement process based on the prediction. In the configuration thatthe SCR catalyst is placed downstream of the NSR catalyst as describedabove with reference to FIG. 8, a modification may predict whetherin-cylinder rich control for the purpose of supplying NH₃ to the SCRcatalyst is performed in the predetermined time period and control theexecution timings of the sensor recording process and the measurementprocess based on the prediction. More specifically, when the NH₃adsorption amount of the SCR catalyst is greater than a predeterminedNH₃ adsorption amount (allowable adsorption amount) that is larger thanthe lower limit adsorption amount described above, the ECU 6 predictsthat in-cylinder rich control for the purpose of supplying NH₃ to theSCR catalyst is not performed in the predetermined time period. When theNH₃ adsorption amount of the SCR catalyst is equal to or less than theallowable adsorption amount, on the other hand, the ECU 6 predicts thatin-cylinder rich control for the purpose of supplying NH₃ to the SCRcatalyst is performed in the predetermined time period. The “allowableadsorption amount” is provided as an NH₃ adsorption amount that causesin-cylinder rich control for the purpose of supplying NH₃ to the SCRcatalyst to be performed in the predetermined time period in the casewhere the sensor recovery process and the measurement process areperformed in the state that the NH₃ adsorption amount of the SCRcatalyst is equal to or less than the allowable adsorption amount, andmay be determined in advance by a fitting operation based on anexperiment or the like.

The following describes a procedure of failure diagnosis processaccording to this modification with reference to FIG. 12. FIG. 12 is aflowchart showing a processing routine performed by the ECU 6 todiagnose a failure of the particulate filter. The like steps to those ofthe processing routine of FIG. 10 are expressed by the like step numbersin FIG. 12.

In the processing routine of FIG. 12, in the case of an affirmativeanswer at S102, the ECU 6 proceeds to S401, instead of S201. At S401,the ECU 6 determines whether the NH₃ adsorption amount of the SCRcatalyst (Anh3 in FIG. 12) is greater than the allowable adsorptionamount (Anh3t in FIG. 12). The NH₃ adsorption amount of the SCR catalystmay he determined by computing a balance between the amount of NH₃consumed by, for example, reduction of NO_(X) by the SCR catalyst andthe amount of NH₃ produced by the NSR catalyst.

In the case of an affirmative answer at S401, it is predicted that thein-cylinder rich control for the purpose of supplying NH₃ to the SCRcatalyst is not performed in the predetermined time period. The ECU 6accordingly proceeds to S202 to start the sensor recovery process. Inthe case of a negative answer at S401, on the other hand, it ispredicted that the in-cylinder rich control for the purpose of supplyingNH₃ to the SCR catalyst is performed in the predetermined time period.The ECU 6 accordingly proceeds to S205 to perform the sensor recoveryprocess according to the execution timing of the in-cylinder richcontrol.

As described above, in the diagnosis of a failure of the particulatefilter by the processing routine of FIG. 12, on prediction that thein-cylinder rich control for the purpose of supplying NH₃ to the SCRcatalyst is performed in the predetermined time period, the sensorrecovery process is performed according to the execution timing of thein-cylinder rich control. This enables the measurement process to beperformed at an earlier time after termination of the in-cylinder richcontrol and thereby prevents the in-cylinder rich control from beingperformed in the predetermined time period. On prediction that thein-cylinder rich control for the purpose of supplying NH₃ to the SCRcatalyst is not performed in the predetermined time period, on the otherhand, the sensor recovery process and the measurement process areperformed immediately. This ensures quick detection of a failure of theparticulate filter.

The failure diagnosis process described in this modification is alsoapplicable to the case where a three-way catalyst, instead of the NSRcatalyst, is placed in the catalyst casing 50. In this modifiedconfiguration, when the NH₃ adsorption amount of the SCR catalystdecreases to or below the allowable adsorption amount described above,in-cylinder rich control is performed for the purpose of producing NH₃by the three-way catalyst,

Upon satisfaction of at least one of the conditions that the NO_(X)storage amount of the NSR catalyst is equal to or greater than theallowable storage amount, that the sulfur poisoning amount of the NSRcatalyst is equal to or greater than the allowable poisoning amount, andthat the NH₃ adsorption amount of the SCR catalyst is equal to or lessthan the allowable adsorption amount, the ECU 6 may predict that thein-cylinder rich control is performed in the predetermined time period.

While disclosed embodiments have been described with reference toexemplary embodiments, it is to be understood that the embodiments arenot limited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2015-058558, filed on Mar. 20, 2015, which is hereby incorporated byreference herein in its entirety.

REFERENCE SIGNS LIST

1 internal combustion engine

2 cylinder

3 fuel injection valve

4 air intake pipe

5 exhaust pipe

6 ECU

40 air flow meter

41 pair intake throttle valve

50 catalyst casing

51 filter casing

52 air-fuel ratio sensor

53 exhaust temperature sensor

54 PM sensor

55 catalyst casing

540 insulator

541 electrode

543 sensor element

544 ammeter

545 heater

546 cover

547 through holes

What is claimed is:
 1. A failure diagnosis device for an emissioncontrol system, wherein the emission control system includes aparticulate filter that is placed in an exhaust conduit of an internalcombustion engine and that is configured to trap PM in exhaust gas; anexhaust gas purification device that is placed upstream of theparticulate filter in the exhaust conduit and that is configured topurify the exhaust gas by utilizing a non-combusted fuel componentincluded in the exhaust gas; and a supplier that is configured toperform in-cylinder rich control of changing an air-fuel ratio of anair-fuel mixture subjected to combustion in the internal combustionengine to a rich air-fuel ratio which is lower than a stoichiometricair-fuel ratio, so as to supply the non-combusted fuel component to theexhaust gas purification device, the failure diagnosis devicecomprising: a PM sensor that is provided to detect an amount of PMflowing out of the particulate filter, the PM sensor having a sensorelement that includes electrodes opposed to each other across aninsulating layer and a heater that is configured to heat the sensorelement, the PM sensor being configured to output an electric signalrelating to an amount of PM depositing between the electrodes underapplication of a predetermined voltage to the sensor element; and acontroller comprising at least one processor configured to perform aprocess of diagnosing a failure of the particulate filter, based on anoutput value of the PM sensor, wherein the controller is programmed to:perform a sensor recovery process that controls the heater to heat thesensor element to a temperature that allows for oxidation of PM andthereby oxidizes and removes the PM depositing between the electrodes,and a measurement process that starts application of the predeterminedvoltage to the sensor element after termination of the sensor recoveryprocess and measures an output value of the PM sensor when apredetermined time period has elapsed since start of application of thepredetermined voltage; and diagnose a failure of the particulate filterby comparing the obtained output value of the PM sensor with apredefined reference value, wherein the controller performs the sensorrecovery process during the in-cylinder rich control or triggered bytermination of the in-cylinder rich control and subsequently performsthe measurement process after termination of the in-cylinder richcontrol and the sensor recovery process.
 2. The failure diagnosis deviceof the emission control system according to claim 1, wherein when thesensor recovery process is triggered by termination of the in-cylinderrich control, the controller performs preheat treatment that controlsthe heater to heat the sensor element to a specified temperature lowerthan the temperature that allows for oxidation of PM during thein-cylinder rich control.
 3. The failure diagnosis device of theemission control system according to claim 1, wherein the controller isfurther programmed to: predict whether the in-cylinder rich control isperformed in the predetermined time period, before the sensor recoveryprocess is actually performed, wherein when it is predicted that thein-cylinder rich control is performed in the predetermined time period,the controller performs the sensor recovery process during thein-cylinder rich control or triggered by termination of the in-cylinderrich control and subsequently performs the measurement process aftertermination of the in-cylinder rich control and the sensor recoveryprocess, and when it is predicted that the in-cylinder rich control isnot performed in the predetermined time period, the controller performsthe sensor recovery process and subsequently performs the measurementprocess after termination of the sensor recovery process.
 4. The failurediagnosis device of the emission control system according to claim 2,wherein the controller is further programmed to: predict whether thein-cylinder rich control is performed in the predetermined time period,before the sensor recovery process is actually performed, wherein whenit is predicted that the in-cylinder rich control is performed in thepredetermined time period, the controller is programmed to perform thesensor recovery process during the in-cylinder rich control or triggeredby termination of the in-cylinder rich control and subsequently performsthe measurement process after termination of the in-cylinder richcontrol and the sensor recovery process, and when it is predicted thatthe in-cylinder rich control is not performed in the predetermined timeperiod, the controller is programmed to perform the sensor recoveryprocess and subsequently performs the measurement process aftertermination of the sensor recovery process.
 5. The failure diagnosisdevice of the emission control system according to claim 3, wherein theexhaust gas purification device includes an NO_(X) storage reductioncatalyst that is configured to store NO_(X) in the exhaust gas when anair-fuel ratio of the exhaust gas is a lean air-fuel ratio higher thanthe stoichiometric air-fuel ratio and to reduce NO_(X) stored in theNO_(X) storage reduction catalyst when the air-fuel ratio of the exhaustgas is a rich air-fuel ratio lower than the stoichiometric air-fuelratio, the supplier performs the in-cylinder rich control to reduceNO_(X) stored in the NO_(X) storage reduction catalyst, when a NO_(X)storage amount of the NO_(X) storage reduction catalyst becomes equal toor greater than a predetermined upper limit storage amount, and thecontroller is programmed to predict that the in-cylinder rich control isperformed in the predetermined time period when the NO_(X) storageamount of the NO_(X) storage reduction catalyst is equal to or greaterthan an allowable storage amount which is smaller than the upper limitstorage amount, while predicting that the in-cylinder rich control isnot performed the predetermined time period when the NO_(X) storageamount of the NO_(X) storage reduction catalyst is less than theallowable storage amount.
 6. The failure diagnosis device of theemission control system according to claim 4, wherein the exhaust gaspurification device includes an NO_(X) storage reduction catalyst thatis configured to store NO_(X) in the exhaust gas when an air-fuel ratioof the exhaust gas is a lean air-fuel ratio higher than thestoichiometric air-fuel ratio and to reduce NO_(X) stored in the NO_(X)storage reduction catalyst when the air-fuel ratio of the exhaust gas isa rich air-fuel ratio lower than the stoichiometric air-fuel ratio, thesupplier performs the in-cylinder rich control to reduce NO_(X) storedin the NO_(X) storage reduction catalyst, when a NO_(X) storage amountof the NO_(X) storage reduction catalyst becomes equal to or greaterthan a predetermined upper limit storage amount, and the controllerpredicts that the in-cylinder rich control is performed in thepredetermined time period when the NO_(X) storage amount of the NO_(X)storage reduction catalyst is equal to or greater than an allowablestorage amount which is smaller than the upper limit storage amount,while predicting that the in-cylinder rich control is not performed inthe predetermined time period when the NO_(X) storage amount of theNO_(X) storage reduction catalyst is less than the allowable storageamount.
 7. The failure diagnosis device of the emission control systemaccording to claim 3, wherein the exhaust gas purification deviceincludes an NO_(X) storage reduction catalyst that is configured tostore NO_(X) in the exhaust gas when an air-fuel ratio of the exhaustgas is a lean air-fuel ratio higher than the stoichiometric air-fuelratio and to reduce NO_(X) stored in the NO_(X) storage reductioncatalyst when the air-fuel ratio of the exhaust gas is a rich air-fuelratio lower than the stoichiometric air-fuel ratio, the supplierperforms the in-cylinder rich control to remove a sulfur component fromthe NO_(X) storage reduction catalyst when a sulfur poisoning amount ofthe NO_(X) storage reduction catalyst becomes equal to or greater than apredetermined upper limit poisoning amount, and the controller predictsthat the in-cylinder rich control is performed in the predetermined timeperiod when the sulfur poisoning amount of the NO_(X) storage reductioncatalyst is equal to or greater than an allowable poisoning amount whichis smaller than the upper limit poisoning amount, while predicting thatthe in-cylinder rich control is not performed in the predetermined timeperiod when the sulfur poisoning amount of the NO_(X) storage reductioncatalyst is less than the allowable poisoning amount.
 8. The failurediagnosis device of the emission control system according to claim 4,wherein the exhaust gas purification device includes an NO_(X) storagereduction catalyst that is configured to store NO_(X) in the exhaust gaswhen an air-fuel ratio of the exhaust gas is a lean air-fuel ratiohigher than the stoichiometric air-fuel ratio and to reduce NO_(X)stored in the NO_(X) storage reduction catalyst when the air-fuel ratioof the exhaust gas is a rich air-fuel ratio lower than thestoichiometric ratio, the supplier performs the in-cylinder rich controlto remove a sulfur component from the NO_(X) storage reduction catalystwhen a sulfur poisoning amount of the NO_(X) storage reduction catalystbecomes equal to or greater than a predetermined upper limit poisoningamount, and the controller predicts that the in-cylinder rich control isperformed in the predetermined time period when the sulfur poisoningamount of the NO_(X) storage reduction catalyst is equal to or greaterthan an allowable poisoning amount which is smaller than the upper limitpoisoning amount, while predicting that the in-cylinder rich control isnot performed in the predetermined time period when the sulfur poisoningamount of the NO_(X) storage reduction catalyst is less than theallowable poisoning amount.
 9. The failure diagnosis device of theemission control system according to claim 3, wherein the exhaust gaspurification device includes a selective catalytic reduction catalystthat is configured to adsorb NH₃ included in the exhaust gas and reduceNO_(X) in the exhaust gas using the adsorbed NH₃ as a reducing agent,and an NH₃ producing catalyst that is placed upstream of the selectivecatalytic reduction catalyst and is configured to produce NH₃ when anair-fuel ratio of the exhaust gas is a rich air-fuel ratio lower thanthe stoichiometric air-fuel ratio, the supplier performs the in-cylinderrich control to produce NH₃ by the NH₃ producing catalyst when an NH₃adsorption amount of the selective catalytic reduction catalyst becomesequal to or less than a predetermined lower limit adsorption amount, andthe controller predicts that the in-cylinder rich control is performedin the predetermined time period when the NH₃ adsorption amount of theselective catalytic reduction catalyst is equal to or less than anallowable adsorption amount which is larger than the lower limitadsorption amount, while predicting that the in-cylinder rich control isnot performed in the predetermined time period when the NH₃ adsorptionamount of the selective catalytic reduction catalyst is greater than theallowable adsorption amount.
 10. The failure diagnosis device of theemission control system according to claim 4, wherein the exhaust gaspurification device includes a selective catalytic reduction catalystthat is configured to adsorb NH₃ included in the exhaust gas and reduceNO_(X) in the exhaust gas using the adsorbed NH₃ as a reducing agent,and an NH₃ producing catalyst that is placed upstream of the selectivecatalytic reduction catalyst and is configured to produce NH₃ when tinair-fuel ratio of the exhaust gas is a rich air-fuel ratio lower thanthe stoichiometric air-fuel ratio, the supplier performs the in-cylinderrich control to produce NH₃ by the NH₃ producing catalyst when an NH₃adsorption amount of the selective catalytic reduction catalyst becomesequal to or less than a predetermined lower limit adsorption amount, andthe controller predicts that the in-cylinder rich control is performedin the predetermined time period when the NH₃ adsorption amount of theselective catalytic reduction catalyst is equal to or less than anallowable adsorption amount which is larger than the lower limitadsorption amount, while predicting that the in-cylinder rich control isnot performed in the predetermined time period when the NH₃ adsorptionamount of the selective catalytic reduction catalyst is greater than theallowable adsorption amount.